The invention relates to aqueous compounds containing waxes and gypsum products that can be manufactured from these, particularly aerated gypsum and plasterboard.
Gypsum is calcium sulphate that can be present with and without crystallisation water. Naturally occurring gypsum rock is calcium sulphate dihydrate (CaSO4×2H2O); the anhydrous form of calcium sulphate is frequently referred to as anhydrite (CaSO4). In the language of this application, “gypsum” refers to naturally occurring gypsum rock, the corresponding products of industrial processes and also the products obtained during the burning of these raw materials.
Gypsum is ideally suited as a construction and working material, due to the fact that it is easy to dehydrate (dehydration (1)). Dehydration is a reversible process. Exposure to energy drives some or all of the crystallisation water out of the calcium sulphate dihydrate. The reverse reaction after water is added (rehydration (2)) causes the previously burned, i.e. at least partially dehydrated gypsum, to become solid, forming a crystalline structure.(CaSO4×2H2O)+T→(CaSO4×0.5H2O)+1.5H2O(CaSO4×2H2O)+T→(CaSO4×2H2OGypsum stone+energy→burnt gypsum  (1)Gypsum stone+energy→burnt gypsum  (2)
Naturally occurring gypsum varies in terms of its purity. Natural impurities include, e.g. limestone (e.g. muscovite or dolomite CaCO3×MgCO3), marl, mineral clay (e.g. montmorillonite or caolinite), and occasionally also sand, bitumen or a variety of salts. However, gypsum is also accessible from various technical processes. For example, flue-gas or REA gypsum with a high degree of purity is produced during the desulphurisation of flue gases.
Gypsum plasterboards are industrially manufactured building boards essentially made up of gypsum, the surfaces and possibly also the longitudinal edges of which are surrounded by firmly adhered paperboard to suit the application. The gypsum core surrounded by paperboard may contain air spaces and additives to achieve given properties.
Essential mechanical plasterboard properties result from the combined effect of the gypsum core and the paperboard casing. This involves the paperboard acting to reinforce the tension area, which in conjunction with the gypsum core gives the gypsum plasterboard the required strength and flexural rigidity. Various types of gypsum plasterboard (GKB) are distinguished, e.g. those that include additives to delay water absorption (impregnated gypsum plasterboard (GKBI) and impregnated fire-resistant gypsum plasterboard (GKFI)).
The water absorption and drying-out time of gypsum plaster-board is tested according to DIN 18180 (2 hours' spent in water).
GKB/GKFGKBI/GKFIWater absorption in (w/w) %30-50<10Drying-out time in hrs7015
Gypsum plasterboard is made from gypsum and additives for the gypsum core, as well as high-grade, repeatedly couched paperboard on large belt systems running continuously. Plaster of Paris (low-fired gypsum, produced at temperatures of roughly 120° C. to 180° C.) is frequently used. The production process comprises the following steps:                paperboard supplied at the bottom,        gypsum slurry supplied and distributed by the moulding station while paperboard is simultaneously supplied from above to form the top layer,        hardening section,        insertion in a dryer (usually after being divided up into individual boards) and        delivery and possible trimming of the transverse edges and bundling of the boards.        
In addition, there are also gypsum building materials in the form of wallboards made from gypsum, which are briefly referred to as “gypsum wallboards”. These are factory-made building panels made from plaster of Paris and water for non-weight-bearing structural components. Gypsum wallboards may contain fibres, fillers and additive substances, as well as other additives, and they may be coloured with pigments to distinguish them visually. They have smooth visible surfaces and are designed with alternating tongue and groove finishes on the abutting and storage surfaces. Here, too, water-repellent boards are known.
Board production takes place in largely automated production plants. Uniform, runny slurry is made from plaster of Paris, water and possibly additives in a dosage and mixing apparatus and used to fill the moulding boxes of the gypsum plasterboard machine. Once the mixture has set, the boards are ejected hydraulically and transported to the dryer. This is followed by drying in heated continuous driers. After drying, the boards are combined into packages or loaded onto pallets.
Gypsum fibre boards are furthermore referred to as gypsum-based plasterboard.
The production of gypsum with low bulk densities, particularly so-called foam or aerated gypsum, is the object of many patents and publications. The solutions known hitherto can generally be divided into two groups:
(1) Use of gas formers (propellants), which are added to the binding agent partially dry or are completely or partially added to the latter only during the aerated gypsum production process via the mixing water. The gas formers mainly comprise several material components in which a chemical reaction is activated by the water coupled with the creation of a gas. The resulting gas bubbles force the gypsum slurry up in the mould.
(2) Undermixing of separately prepared foam, which is added to the mixture in the mixer and immediately causes pores to form in the made-up material slurry. Foam is mainly produced by atomising water and air enriched with surface-active substances (surfactants).
It is also known that aerated gypsum elements produced using the aforementioned method are additionally water-repellent. This involves water-repellent additives being added to the mixer in the mixture preparation phase.
Gypsum products, particularly gypsum plasterboard, are provided with additives that delay water absorption, for use in damp areas. The impregnating additives are usually added to the gypsum mixture before the gypsum products are made and worked into it uniformly, after which the gypsum mixtures are usually dried in layers at a high temperature of e.g. 100 to 150° C. and then cooled. If the gypsum boards are covered with paperboard, they are referred to as gypsum plasterboards.
If the gypsum does not receive suitable water-repellent treatment, the effect of dampness can cause the paperboard to soften, leading to deformation. The use of gypsum plasterboard in damp rooms, such as bathrooms, laundry rooms, etc. is therefore problematic due to its diminishing rigidity and deformation. This has particularly serious effects when the gypsum plasterboard has ceramic tiles adhered to it, for example. The dampness causes the paperboard to soften, lose its inner rigidity and split under the weight of the ceramic tiles, causing the layer of paperboard attached to the boards to fall away. The destruction of the remaining gypsum paperboard is then only a matter of time.
In addition to the diminishing rigidity and deformation, corrosion and mildew can accelerate the destruction. Even the adhesive layer applied to the entire surface for the ceramic tiles, which usually has a synthetic base, cannot inhibit the effect of the dampness.
The use of silicon or siloxane emulsions/dispersions for impregnation is known. In addition, dispersions based on paraffin or montan waxes are known. However, polymers and resins are also used, which do not fall into the wax category. Polyvinyl alcohols are referred to as polymers. The wax components are in some cases also added to the gypsum mixture in powder form. Mixtures of asphalt or bitumen dispersions, in some cases used in combination with polyvinyl alcohols, in which a silicon compound or a synthetic resin emulsion has been added to the gypsum, are likewise known from the literature.
However, it is difficult to obtain a satisfactory water-repellent or hydrophobic product, which is sufficiently hydrophobic and, at the same time, facilitates the necessary foam stability to produce an aerated product.
U.S. Pat. No. 3,935,021 describes a gypsum wallboard in which polyvinyl alcohol and a wax-asphalt emulsion are introduced into the gypsum core. Wax-asphalt emulsions are widely used in gypsum wallboards, although certain disadvantages are associated with their use.
In U.S. Pat. No. 5,437,722 an aqueous emulsion is used to make gypsum products water-repellent, which comprises a hydrocarbon wax, a montan wax and an emulsifier/stabiliser system with the addition of polyvinyl alcohol.
WO 98/09925 describes an aerated gypsum product, which is made water-repellent through the inclusion of an aqueous emulsion, which comprises a hydrocarbon wax, a montan wax and a colloid-stabilised emulsifier system.
Other aqueous wax dispersions for making gypsum products water-repellent, which contain starch, long-chain alkyl phenols, saponified montan waxes, surfactant, complexing agents and paraffin waxes are known from U.S. Pat. No. 6,585,820. U.S. Pat. No. 6,595,553 also mentions surfactant, montan wax and paraffin wax as an integral part of the aqueous wax dispersion.
The problem addressed by the present invention is that of finding a water-repelling agent that is suitable for both traditional gypsum plasterboard and also for the technically more complex production of aerated gypsum. A further problem facing the invention is that of guaranteeing sufficient vapour permeability, despite the water-repellent nature. Furthermore, the wax additive must improve the flow performance of the gypsum mixture, bring about a slight change in the reinforcing behaviour of the gypsum, cause a significant improvement in the paperboard adhesion and have a very slight effect on foaming in the process.
The effectiveness of wax dispersions in gypsum products is crucially dependent on the composition. It is obviously important for the internal surface of the micropores to be sealed in the gypsum. On the other hand, the pores must not be completely closed off, in order to retain the desired vapour permeability. This cannot be achieved with pure hydrocarbon dispersions according to our findings. In the past, montan wax was used here in the mixture with paraffin. Montan wax is a hard fossil wax of vegetable origin, which has survived the carbonisation process virtually unchanged. It therefore occurs in some brown coal as an extractable element. Important chemical parameters include the acid number (AN) and saponification number (SN) of the montan wax. Furthermore, a small number of hydrocarbons are contained in montan waxes, but not unsubstantial proportions of montan resins and asphalt substances, whereby the latter contain a not unsubstantial proportion of inorganic ash components (0.4 to 4% by wt.).
Montan resins and ash components may affect the colour if, for example, a particularly light-coloured gypsum is required when the material is used in visible construction work. The hydrocarbon chains in the wax acids and wax alcohols have a chain length of 20 to 34 C units with a maximum of around 30 C units. In this case, the focus lies on chains with a straight-line number of carbon atoms.
When using wax dispersions in impregnated gypsum plasterboard (GKBI) and impregnated fire-resistant gypsum plasterboard (GKFI), it is generally of particular significance that these do not have a negative effect on the paperboard's adhesion to the surface of the gypsum core. Furthermore, the emulsifier systems must also be based on optimum adhesion of the paperboard layer and minimum interference with the foaming behaviour of the gypsum slurry. Surprisingly, it was found that the water-repelling effect in gypsum products known hitherto predominantly from wax dispersions containing montan wax can also be achieved with other specially selected polar synthetic and natural components. Apart from paraffin and in claim 1 obligatory named polar long-chain compounds, longer-chain alphaolefins (hydrated and/or unhydrated) and Fischer-Tropsch waxes can also be used as the basic wax phase. Also capable of being used in the composition are natural waxes, fats, fatty alcohols and synthetic and synthetically modified natural resin components.
It emerged that the desired water-repellence could also be achieved with the help of wax phases without montan wax. In this case, other wax phases and also other polar additives were identified as useable. This produces lighter/whiter gypsum products that can also be used for visible construction, as the wax phases, particularly the additives used, display significantly lower ash contents or none at all and also contain no dark asphalt and resin components.